Recent results have shown that the secret-key rate of coherent-one-way (COW) quantum key distribution (QKD) scales quadratically with the system’s transmittance, thus rendering this protocol unsuitable for long-distance transmission. This was proven by using a so-called zero-error attack, which relies on an unambiguous state discrimination (USD) measurement. This type of attack allows the eavesdropper to learn the whole secret key without introducing any error. Here, we investigate the feasibility and effectiveness of zero-error attacks against COW QKD with present-day technology. For this, we introduce two practical USD receivers that can be realized with linear passive optical elements, phase-space displacement operations and threshold single-photon detectors. The first receiver is optimal with respect to its success probability, while the second one can impose stronger restrictions on the protocol’s performance with faulty eavesdropping equipment. Our findings suggest that zero-error attacks could break the security of COW QKD even assuming realistic experimental conditions.

中文翻译：

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利用当今技术破解相干单向量子密钥分发


最近的结果表明，相干单向（COW）量子密钥分发（QKD）的密钥率与系统的传输率呈二次方关系，因此使得该协议不适合长距离传输。这是通过使用所谓的零错误攻击来证明的，该攻击依赖于明确的状态歧视（USD）测量。这种类型的攻击允许窃听者获知整个密钥而不会引入任何错误。在这里，我们研究了利用现有技术对 COW QKD 进行零错误攻击的可行性和有效性。为此，我们介绍了两种实用的 USD 接收器，可以通过线性无源光学元件、相空间位移操作和阈值单光子探测器来实现。第一个接收器就其成功概率而言是最佳的，而第二个接收器可以通过有故障的窃听设备对协议的性能施加更严格的限制。我们的研究结果表明，即使假设现实的实验条件，零错误攻击也可能破坏 COW QKD 的安全性。